I managed quite well down there. I had the meteor armor and the rocket boots, mind. So in the open areas it's pretty easy to avoid them.
Also the meteorite hamaxe does wonders on them. And if you're at the right angle, their shots get deflected by the pick. At least the nightmare pick did for me.
Now I have full molten armor and a molten pick.
Still haven't fully explored the underground jungle or even tried beating Skeletron.